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It sounds like this would be a switch for the electric mode (120 vac) operation for the water heater. Mine is located at the end of the kitchen cabinet and controls the receptacle the water heater cord is plugged into.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7